Medical devices comprising a coil, such as guide wires and catheters, are generally formed of stainless steel and may comprise a plastic material, such as a polyurethane, polyamide, or polyyolefin, etc. Some medical devices, such as guide wires having a coil tip, may comprise a jacket or sleeve of a polymeric material that is coated or extruded over the coiled portion so as to provide a surface to which any subsequent coatings can adhere, as well as to provide smoothness and uniformity to the surface.
In certain medical procedures, a catheter is typically inserted at a predetermined site, and a guide wire tip is inserted through the catheter so that the coil tip is protruding from the catheter. Then, the catheter with the guide wire is inserted percutaneously into the targeted body part, such as a blood vessel, and the catheter is further inserted through the vessel by using the guide wire as a leading and supporting guide. These operations produce friction and abrasive forces that apply to the surfaces of the medical device. Relatively high friction between the catheter and the guide wire not only prevents the guide wire from being inserted through the catheter, but also prevents the guide wire from being easily moved through the catheter, thus making it difficult to carry out subtle indwelling operations at the targeted vessel site. Sometimes, the guide wire cannot be withdrawn from the catheter, rendering the catheter lumen unusable despite the completion of the indwelling operation. This higher level of friction presents an issue to the design and construction of such devices, as a low level of frictional resistance between the catheter's inner surface and the guide wire is desirable in the intended applications.
Attempts have been made to apply low frictional resistance Teflon and silicone oil to the outer surface of guide wires in order to overcome such problems. However, application of silicone oil fails to retain lubricity because of immediate loss of silicone coatings. Further, frequent applications add to frictional resistance, also undesirably creating the problems mentioned above.
In certain guide wires, the flexibility of the tip of the guide wire is created by having a very thin core wire over which a very fine coil wire is wound to make a flexible tip. For example, a core wire for a 0.014 inch (in.) guide wire may taper down to a 0.010 in. and be coiled with a wire of dimension 0.002 in. to create a 0.014 in. tip. The flexibility of the tip is dependent upon these coils being able to flex independent of each other and not be bound together. Various coatings have been applied to coiled tip guide wires or catheters to obtain a flexible distil tip of the wire or catheter. However, most coatings involve cross-linking and/or covalent binding to the device or a portion thereof. Thus, such coatings often form a film over the loop portions of the coil and lock them together, thereby creating a stiff tip. Also, when these wires are used in the body, the coil eventually flexes and breaks the coating into fragments. This often results in undesirable pieces being broken off in the body, as well as yielding a ragged or jagged broken coating.
Hydrophilic polyurethane coatings have been applied directly on metal surfaces. However, commercial versions of this technology require cross-linking and thick layers (60-80 microns thick) in order to achieve adequate performance. In practice, the thick layer extends continuously around the coated metal substrate. These layers have good cohesive forces and thus appear to be tightly bound on the metal surface, even though these layers do not necessarily have good adhesion to the metal surface. One disadvantage of such coatings is that because the polyurethane and other plastic layers are so thick, the diameter of the underlying wire must be correspondingly diminished. This is especially troublesome on very fine wires, such as those used in coronary angioplasty or neurointerventional catheterization procedures. These wires have outer diameters of about 0.010 in. to about 0.014 in. Alternatively, low frictional materials, such as polytetrafluoroethylene coatings, have been used. Such materials have lower friction than metals and most other plastic materials, and they can be applied directly onto metallic substrates. Other materials, such as high density polyethylene have been tried, but the coefficients of friction are not low enough for these materials to be useful. Oils have been applied; although their coefficients of friction are low, such treatments are transient as they wear off during use.
Hydrogel coatings are known to provide a lubricious surface for insertable devices. However, metals and certain plastic materials, such as polyolefins, polyamides, silicones, polyesters, have inert surfaces and it is often difficult to achieve acceptable adhesion when applying coatings, including hydrogel coatings, over such surfaces.
A variety of lubricious or “slippery” coatings have been proposed for use on biomedical devices, such as catheters, guide wires, endotracheal tubes and implants, in order to reduce friction when the device is introduced through tissue, or into blood vessels, or into other parts of the anatomy. Common materials used in the art to provide lubricious coatings for biomedical devices include oil, silicone and polymeric materials, such as poly N-vinylpyrrolidone, hydrophilic polyurethanes, polyethylene oxide and polyacrylic acid. Among the most common materials used to provide lubricious coatings are hydrophilic polymers which are covalently bonded to the substrate with a binder polymer having reactive functional groups, e.g., isocyanate, aldehyde and epoxy groups. Although the use of such binder polymers having reactive functional groups is effective in providing lubricious coatings that have a high degree of abrasion resistance, such binder polymers are often highly reactive, toxic, require solvents, and typically require special handling techniques in order to avoid potential health, safety and environmental problems. Further, the polymeric coating often contains cross-linkers, which generally contain hazardous or potentially hazardous materials, such as isocyanates or aziridines. Moreover, the cross-linked polymers yield a rigid coating that makes the coil of the wires stiff and non-trackable.
Accordingly, there is a need for medical devices, such as coiled guide wires, having a lower frictional resistance surface which enables more subtle operation in a vessel, and which can be easily inserted into a catheter, and for new or improved and less toxic coating processes to facilitate such devices.